Add Accept header validation

- Set Accept header to request JSON or SSE responses
- Validate response Content-Type and raise error for non-JSON responses
- Add Accept header to test fixtures
- Use explicit hash syntax for Ruby 2.7 compatibility

lib/mcp/client/http.rb

@@ -3,6 +3,8 @@
 module MCP
   class Client
     class HTTP
+      ACCEPT_HEADER = "application/json, text/event-stream"
+
       attr_reader :url
 
       def initialize(url:, headers: {})
@@ -14,46 +16,48 @@ module MCP
         method = request[:method] || request["method"]
         params = request[:params] || request["params"]
 
-        client.post("", request).body
+        response = client.post("", request)
+        validate_response_content_type!(response, method, params)
+        response.body
       rescue Faraday::BadRequestError => e
         raise RequestHandlerError.new(
           "The #{method} request is invalid",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :bad_request,
           original_error: e,
         )
       rescue Faraday::UnauthorizedError => e
         raise RequestHandlerError.new(
           "You are unauthorized to make #{method} requests",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :unauthorized,
           original_error: e,
         )
       rescue Faraday::ForbiddenError => e
         raise RequestHandlerError.new(
           "You are forbidden to make #{method} requests",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :forbidden,
           original_error: e,
         )
       rescue Faraday::ResourceNotFound => e
         raise RequestHandlerError.new(
           "The #{method} request is not found",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :not_found,
           original_error: e,
         )
       rescue Faraday::UnprocessableEntityError => e
         raise RequestHandlerError.new(
           "The #{method} request is unprocessable",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :unprocessable_entity,
           original_error: e,
         )
       rescue Faraday::Error => e # Catch-all
         raise RequestHandlerError.new(
           "Internal error handling #{method} request",
-          { method:, params: },
+          { method: method, params: params },
           error_type: :internal_error,
           original_error: e,
         )
@@ -70,6 +74,7 @@ module MCP
           faraday.response(:json)
           faraday.response(:raise_error)
 
+          faraday.headers["Accept"] = ACCEPT_HEADER
           headers.each do |key, value|
             faraday.headers[key] = value
           end
@@ -83,6 +88,17 @@ module MCP
           "Add it to your Gemfile: gem 'faraday', '>= 2.0'" \
           "See https://rubygems.org/gems/faraday for more details."
       end
+
+      def validate_response_content_type!(response, method, params)
+        content_type = response.headers["Content-Type"]
+        return if content_type&.include?("application/json")
+
+        raise RequestHandlerError.new(
+          "Unsupported Content-Type: #{content_type.inspect}. This client only supports JSON responses.",
+          { method: method, params: params },
+          error_type: :unsupported_media_type,
+        )
+      end
     end
   end
 end